i3 is a tiling window manager, completely written from scratch.
i3 was created because wmii, our favorite window manager at the time,
didn't provide some features we wanted (multi-monitor done right,
for example), had some bugs, didn't progress since quite some time
and wasn't easy to hack at all (source code comments/documentation
completely lacking). Still, we think the wmii developers and
contributors did a great job.
http://i3wm.org/